How to cut a cube in half
It is fairly simple, but it does utilize the obscure Rip tool (V). Here are the steps:


Tab into Edit Mode.
Key Ctrl + R and move the cursor around until the pink suggested loop cut is dividing the cube in half in the manner that you want it to (X, Y, or Z).
 LMB, then immediately  RMB to add the loop to the actual mesh. Make sure it remains selected; nothing else should be selected.
Key V to rip the vertices (opposite of Alt + M / W > Remove Doubles) and once again immediately  RMB. You should now have two identical copies of the loop cut that you previously added in the same place and one of them selected.
Key Ctrl + L to select the rest of one half of the cube.
Key P to open the Separate menu and select By Selection.
Tab out of Edit Mode. You should now have two identical halves of a cube.


The Procedure:



Edit (Optional Steps 6.5, 8, 9, and 10):

&nbsp;&nbsp;6.5. Use Alt +  RMB to select the loop cut on the remaining half of the cube and key F to create a face.

&nbsp;&nbsp;8. Select the other cuboid half and Tab into Edit Mode.

&nbsp;&nbsp;9. Use Alt +  RMB to select the loop cut on the remaining half of the cube and key F to create a face.

&nbsp;&nbsp;10. Tab out of Edit Mode.

Steps 6.5-10: